1. Field of Invention
This invention relates to systems and methods for reducing or eliminating hue shifts contours, and other undesirable artifacts in an output image.
2. Description of Related Art
Digital reproduction, transfer or display of various images can occur using a variety of image processing devices and systems in a variety of environments. A source image may be input into a device, processed in some manner, and then output for reproduction or printing as a color output image. The generation of color output image can be thought of as a two step process. In the first step, Red, Green, Blue (RGB) image signals representative of the input image are produced by, for example, a scanner or a work station on a CRT display. Thereafter, a printer can receive the RGB image signals, convert them to Cyan, Magenta, Yellow, Key or Black (CMYK) printer signals and, if desired, generate a hard copy reproduction in accordance thereto. Many printers used in the art have uniquely defined colorants that effect print capabilities. As a result, a select RGB image signal, when converted into a CMYK printer signal, may produce colors that are different to some degree. Each individual printer can be provided with unique multidimensional look-up tables that have been built to convert the RGB image signals into proper CMYK printer signals.
The printer can print a limited range of colors, or xe2x80x9cgamut,xe2x80x9d due to the physical limitations of the marking process. A colorimetric rendering should accurately reproduce any in gamut color. It should also do something sensible for out of gamut colors. While the calorimetric intent is optimized for spot color, it should still operate gracefully for images, which may contain sweeps possibly crossing the gamut boundary. Hence smoothness of sweeps is an important consideration in the design of the algorithm. The colorimetric intent achieves these criteria by using a data extrapolation algorithm for all colors in gamut and near the gamut surface, and an explicit gamut mapping algorithm for all colors that are far from the gamut surface. The gamut mapping is done using an algorithm that produces the printable color that is nearest to the requested color of the same hue (in hue-corrected CIELAB space). A linear blend is then done between the two algorithms to ensure a smooth transition from in-gamut to out-of-gamut regions. Relative colorimetry is used for calorimetric reproduction of in-gamut colors.
As well known in the art, color may be specified in coordinate systems. For example, color may be measured in terms of three components, L* which roughly corresponds to a lightness-darkness scale, a* which roughly corresponds to a red-green scale, and b* which roughly corresponds to a yellow-blue scale. An ink limit function may be used for building the multidimensional lookup tables to convert, for example, from the L*a*b* coordinates to the CMYK coordinates. The multi-dimension lookup tables can be built and applied as part of the conversion process.
In some applications, it may be necessary or desirable to convert an inputted image between the input and the output of one image device for the specific purpose of using the converted image data by at least one other image device. In other applications, it may be necessary or desirable to convert the input image for some particular application within an image device itself. During the conversion process, it may be desirable or necessary to maintain the hues, that is the ratio of the various colors in the image, such CMYK.
In order to reproduce an output image with high quality after the conversion process, it is preferable that the total color area coverage does not exceed the limit of the image output terminal. That is, when the total color area coverage exceeds the limit of the image output terminal, the various color values need to be decreased. However, decreasing the color values by equal amounts may produce kinks, or artifacts, in the image output terminal response. That is, if the one of the colors reaches zero in quantity, and the remaining colors are reduced by equal amounts, kinks in the image output terminal response may occur which may cause contours, undesirable hue shifts and other artifacts which ultimately deteriorate the image data.
FIG. 1 shows one exemplary three-dimensional color plot 100 of a system characteristic curve. As shown in FIG. 1, as the amount of black increases and the total area coverage allowed, that is, the ink limit, decreases, the color values, i.e., cyan, magenta or yellow values are reduced. The solid line 102 in FIG. 1 shows the system response characteristic obtained if CMY values are reduced by equal amounts. As shown in FIG. 1, there are abrupt changes, or xe2x80x9cbending,xe2x80x9d in the slope of the solid line as one of the colorants goes to zero, and thus, as the CMY values approach the white point, kinks may result, for example, in the L*a*b* to CMYK conversion process.
If there are kinks in the L*a*b* to CMYK conversion process, interpolation between the nodes in the multidimensional lookup tables from the L*a*b* coordinates to the CMYK coordinates can give inaccurate results. Moreover, these errors can produce unpleasant artifacts such as hue shifts and contours in the images.
Artifacts stemming from these kinks in the L*a*b* to CMYK conversion process can be perceptible to the human eye, and thus are unacceptable for an output image. That is, if the image is output without adjusting for these kinks, the output image will include the artifacts. These artifacts, even if only a few mils or tens of microns in size, are well within the visual acuity of the human eye. Since the human eye can sense these artifacts, the quality of the resulting image suffers greatly even for small artifacts.
Accordingly, there is a need for systems and methods that reduce or eliminate unacceptable hue shifts and contours in an output image caused by kinks, or bending, that occurs during the interpolation between the nodes in the multidimensional lookup tables from converting the L*a*b* coordinates to the CMYK coordinates.
In the various exemplary embodiments of the systems and methods of this invention, kinks, or bending that occurs during the ink limit conversion process at the ink limit are reduced or eliminated.
In the various exemplary embodiments of the systems and methods of this invention, an adjusted ink limit conversion process is provided which approaches the limit in a smooth fashion.
In the various exemplary embodiments of the systems and methods of this invention, extrapolated values in the input, values that are outside of a predetermined range, are handled in a smooth fashion.
In the various exemplary embodiments of the systems and methods of this invention, extrapolated values are used to avoid kinks in the L*a*b* to CMYK conversion process at the end bounds of the range.
In the various exemplary embodiments of the systems and methods of this invention, ink limiting is performed gradually as the system approaches the ink limit, avoiding sudden changes in the slope of the system response characteristics.
In the various exemplary embodiments of the systems and methods of this invention, if one of a plurality of colors reaches zero ink, the remaining colors are decreased by multiplying them by a factor less than one. This maintains the colorant ratio, and reduces hue shifts.
As shown in FIG. 1, the dashed curve is an example of the system response characteristic achieved by the various exemplary embodiments of this invention. As shown in FIG. 1, the dashed curve changes direction in a smooth way, and hue is preserved as it approaches the white point.